Process for coloring aluminum and alloys of aluminum having an anodized surface

ABSTRACT

THE PRODUCTION OF CORROSION RESISTANT REPRODUCIBLE COLORS ON THE SURFACE OF ALUMINUM AND ALLOYS OF ALUMINUM WHEREIN AN ALUMINUM OXIDE LAYER IS FORMED ON THE SURFACE BY ANODIZATION AND THE COLORED PARTICLES OF A METAL OR COMPOUND OF A METAL ARE DEPOSITED IN THE POROUS SURFACE BY ALTERNATING CURRENT ELECTROLYSIS WITH THE OBJECT MOUNTED AS AN ELECTRODE IN AN ELECTROLYTE CONTAINING THE CONSTITUENT METAL AND WHEREIN THE OBJECT WITH THE COLORED PARTICLES DEPOSITED THEREIN IS SUBJECTED TO ANODIC TREATMENT IN AN ELECTROLYTE CONTAINING AN ALKALI METAL OR AMMONIUM THIOSULPHATE, WITHOUT AND PREFERABLY WITH SUBSEQUENT SEALING.

United States Patent 3,795,590 PROCESS FOR COLORING ALUMINUM AND ALLOYS0F ALUMINUM HAVING AN ANODIZED SURFACE Jos Patric, Grenoble, France,assignor to Cegedur GP, Paris, France -No Drawing. Filed Dec. 18, 1969,Ser. No. 886,330 Claims priority, applicatigog9France, Dec. 23, 1968, 170 The portion of the term of the patent subsequent to May 23, 1989, hasbeen disclaimed Int. Cl. C23!) 9/02; C23f /00, 17/00 U.S. Cl. 204-35 N24 Claims ABSTRACT OF THE DISCLOSURE This invention relates to aluminumand alloys of aluminum having a protective colored coating and to amethod for producing same.

In the copending application filed on or about Dec. 3, 1969, entitledObjects of Aluminum and Alloys of Aluminum Having Colored Coatings andProcess, description is made of a process for obtaining clear, uniformand reproducible colored coatings formed on anodized aluminum or alloysof aluminum by anodic dissolution of a more darkly colored coatingformed of colored particles deposited by an alternating currentelectrolysis. In the described process, it is possible to effect thedesired dissolution in the same bath which was used for alternatingcurrent coloring.

In contrast, the process of this invention makes use successively of twodifferent electrolysis baths whereby new colors are capable of beingobtained while retaining all of the advantages of the process previouslydescribed. For example, with the previously described process, the useof copper salts in the coloring bath operates to produce colors rangingfrom pink to copper red to bright red whereas the process of thisinvention makes it possible to obtain additional colors such as brightyellow and brick color with the same copper salts.

Thus it is an object of this invention to produce and to provide amethod for producing colored coatings on aluminum and alloys of aluminumin which colors of a greater variety can be obtained, in which coloredcoatings of greater stability and color intensity can be obtained, inwhich the colors can be reproduced in a manner to provide coloredaluminum products having uniform color of high weather resistance and inwhich such colors can be produced in a simple, economical and eflicientmanner.

Briefly described, in accordance with the process of this invention, thesurface of the aluminum or alloy of aluminum to be colored is firstprovided in the conventional manner with a porous layer of aluminumoxide, as by anodization in an acid bath, preferably a sulphuric acid.Colored particles of the metal or metal compound are deposited into theportion of the porous layer by alternating current electrolysis in whichthe aluminum or alloy of aluminum is suspended as an electrode in theelec- 3,795,590 1C Patented Mar. 5, 1974 trolysis bath containing a saltof the metal of the compound dissolved therein in amounts of at least.15 g./ liter to an amount to form a saturated solution and preferablyin an amount of .3 to 30 g./liter to provide a provisional or temporarycolor effect. The other electrode, referred to as the counter-electrode,is composed either of the constituent metal of the salt dissolved in theelectrolysis bath or of a material which is resistant to attack by thebath and does not give rise to secondary reactions during alternatingcurrent electrolysis, such as represented by stainless steel.

The aluminum object containing the temporary colored coating, which hasbeen deposited by alternating current electrolysis, is then subjected toanodic treatment by mounting the object as an anode in an electrolysisbath through which direct current or rectified unfiltered current ispassed. In the practice of the invention, the electrolysis bath which isemployed in the anodic treatment with direct current electrolysiscomprises an aqueous solution containing sodium thiosulphate in anamount within the range of 1 to 100 g./liter and preferably 5 to 20g./liter. Instead of sodium thiosulphate, ammonium thiosulphate or otheralkali metal thiosulphate such as potassium thiosulphate may be used.

As the cathode, use can be made of platinum, stainless steel, or a metalsuch as copper or nickel. The eflfect of anodic treatment is to modifythe temporary color finish. The anodic treatment is carried out atsubstantially constant current density within the range of 0.1 to 5a./dm. and preferably within the range of 0.1 to 1 a./dm. or at asubstantially constant voltage within the range of 10 to volts orpreferably within the range of 15 to 30 volts. The electrolyte isemployed generally within the range of 15 to 50 C. and preferably withinthe range of 18 to 25 C. The desired results can be obtained by anodictreatment under the conditions described in a time span of /6 to 30minutes and generally within the range of /2 to 15 minutes. Duringanodic treatment, the temporary color effect lightens and may evendisappear completely if continued for a sufficiently long period oftime. This is indicative of some type of dissolution of the coloredparticles under the anodic treatment described.

The colored finish obtained after anodic treatment can be fixed bysealing in the conventional manner such as by boiling water, preferablycontaining dissolved nickel salts, or by other conventional means suchas varnishing or the like.

Even when the anodic treatment appears completely to eliminate thetemporary color eifect of the previously deposited colored particles ofmetal or metal compound, the sealing nevertheless reveals a new coloreffect, even when the boiling water sealing bath does not contain nickelsalts.

The final color finish obtained is governed both by the temporary coloreffect obtained during the coloring treatment by alternating currentelectrolysis and by duration of the modifying anodic treatment. Theappearance of the final color effect is somewhat related to the surfacequality of the object before the layer of alumina is formed byanodization. An originally brilliant or shiny surface yields colorfinishes which will be brilliant. 0n the other hand, on a dull or matsurface, the final color will appear dull or mat, but it is therefore ofadvantage to modify the surface characteristics of the object beforecoating, such modification being effected by such well known means aspolishing, butfing, satinizing, and the like, depending upon the finalcolor effect that is desired. For example, a mat surface is obtained bycleaning the object for 10 minutes at 50 C. in a soda solution having aconcentration of 50 g./liter, after which the object is Washed withrunning water, followed by immersion in nitric acid (36 B.) and then byfurther washing in running water. A brilliant appearance is obtained bymechanical polishing or by chemical or electrolytic glossing.

The following examples are given by way of illustration, but not by wayof limitation, of the practice of this invention. In the followingexamples, the objects having a surface of aluminum or alloy of aluminumare generally subjected to the following:

(1) a surface preparation by one of the processes described above;

(2) anodization in a sulphuric acid bath;

(3) coloring with alternating current in an acid solution containingsalts of a metal such as copper, nickel, cobalt and the like withcounter-electrodes made of the constituent metal of the salts orstainless steel, at current densities within the range of 0.5 to a./dm.for periods ranging from 1 to 15 minutes; the result will be a black,dark red or deep bronze color finish;

: (4) anoclic treatment to modify the temporary color effect wherein theobject is mounted as an anode of an electrolysis cell through whichdirect current or unfiltered rectified current is passed and in whichthe electrolyte contains sodium thiosulphate and/or other alkali metalor ammonium thiosulphate and then preferably,

though not essentially, by sealing by immersion in boiling water for 20to 45 minutes with the boiling water optionally containing from 0.5 to 2g./liter of nickel salt such as nickel acetate.

EXAMPLE 1 v A 99.5% pure and previously cleaned aluminum sheet isanodized by mounting as an electrode in a bath containing 200 g./literof sulphuric acid for a period of 30 minutes at 20 C. at a currentdensity of 1.5 a./dm. A

copper sulphate in solution for a period of minutes at p 12 volts withthe result that a black color efiect is obtained. The sheet is thenmounted as the anode in an electrolysis cell in which the electrolyte isan aqueous solution of sodium thiosulphate, Na S- O -5H O', having aconcentration of 10 g./liter. Direct current is passed for a perod of 10minutes at a density of 0.4 a./dm. with the temperature of the bathbeing about 20 C.

The sheet changes color to copper red which becomes a brick-like shadeafter 30 minutes treatment in boiling distilled water. The color effectthat is obtained is uniform and resistant to corrosion and toultra-violet rays.

EXAMPLE 2 An aluminum alloy object containing 0.5% of Si and 0.5 of Mgis mechanically surface polished and then anodized in a sulphuric acidbath to produce a porous surface layer of alumina having a thickness of16 -microns. The object is then colored in an electrolysis cell fed withalternating current in which the electrolyte is formulated of an aqueoussolution of 10 g./liter copper sulphate'in sulphuric acid solution.After 3 minutes treatment at 12 volts, an object which is dark red incolor is produced; The object is then mounted as an anode in anotherelectrolysis cell containing 15 g./liter of sodium thiosulphateinaqueous solution. After passing a direct current of 0.2 a./dm. for 15minutes at C., the surface becomes colorless. After 30 minutes inboiling distilled water containing 2 g./liter of nickel acetate, thesection assumes a deep yellow color which is highly uniform andresistant to corrosion and to ultra-violet rays.

EXAMPLE 3 A sheet of aluminum alloy containing 0.6% of Mg is providedchemically with a gloss surface finish and then anodized in aconventional manner in a sulphuric acid bath to provide a porous surfacelayer of alumina having a thickness of 20 microns. It is then mounted asan elec trode in an electrolysis cell fed with alternating current inwhich the electrolyte is an aqueous solution of 3-10% copper sulphate insulphuric acid, until a dark red color effect is obtained. The coloredsheet is then mounted as an anode in another electrolysis cell fed withcontinuous direct current in which the electrolyte is an aqueoussolution of sodium thiosulphate in a concentration of 20 g./ liter.After 1 minute treatment at 0.6 a./dm. and' at 20 C., a colorless sheetis produced. After 40 minutes in boiling distilled water, a brightyellow color develops on the surface which is highly uniform and whichis resistant to corrosion and to ultra-violet rays.

EXAMPLE 4 An object of an aluminum alloy containing 1% Si, 1% Mg and 1%Mn is electrolytically polished to provide a gloss surface finish andthen anodized in a sulphuric acid bath to provide a porous surface layerof alumina having a thickness of 8 microns. The anodized layer is thencolored in a sulphuric acid solution of 15% by weight copper sulphate,using alternating current, until a black color finish is obtained. Thesection is then mounted as the anode of another electrolysis cellcontaining an aqueous solution of ammonium thiosulphate in aconcentration of 12 g./liter. The cell is fed with rectified unfilteredcurrent of 0.2 a./dm. for 10 minutes at 20 C. The color intensity isreduced as by partial dissolution. It takes on a brick-like color aftertreatment in boiling distilled water containing 1 g./liter of nickelacetate. The brick-like color is highly uniform and resistant tocorrosion and to ultra-violet rays.

It will be apparent from the foregoing that there is provided a new andeconomical process for permanently coloring the surfaces of aluminum andalloys of aluminum with a wide variety of colors, many of which have notheretofore been capable of being obtained, and in which the color isuniform and reproducible while being highly resistant to corrosion andto ultra-violet rays.

It will be understood that changes may be made in the details offormulation and operation without departing from the spirit of theinvention, especially as defined in the following claims.

I claim:

1. In a process for color coating objects of aluminum and alloys ofaluminum in which the surface to be colored is provided with a porousanodized aluminum oxide layer, the steps of depositing colored particlesof a metal or a compound of a metal by alternating current electrolysisof the object mounted as an electrode in an electrolysis bath containinga salt of the metal in solution, mounting the object with the coloredparticles as an anode in an electrolysis bath containing a thiosulphatein solution while passing direct or unfiltered current therethrough tomodify the color effect of the colored particles previously deposited.

2. The process as claimed in claim 1 in which the porous aluminum oxidelayer is provided on the surface by anodizing in a sulphuric acid bath.7

3. The process as claimed in claim 1 which includes the step of sealingthe colored coating that is formed.

4-. The process as claimed in claim 3 in which the colored coating issealed with boiling water.

5. The process as claimed in claim 3 in which the colored coating issealed with boiling water containing a nickel salt in solution.

6. The process as claimed in claim 5 in which the nickel salt is in theform of nickel acetate and present in an amount within the range of 0.5to 2 g./liter.

7. The process as claimed in claim 1 in which the thiosulphate is analkali metal or ammonium thiosulphate.

8. The process as claimed in claim 6 in which the thiosulphate ispresent in the electrolysis bath in an amount within the range of 1 tog./liter.

9. The process as claimed in claim 6 in which the thiosulphate ispresent in the electrolysis bath in an amount within the range of 5 to20 g./liter.

10. The process as claimed in claim 1 in which the cathode in theelectrolysis cell wherein the object is mounted as the anode is selectedfrom the group consisting of platinum, stainless steel, copper andnickel.

11. The process as claimed in claim 1 in which the current is passedthrough the electrolysis cell for anodic treatment of the object at acurrent density of 0.1 to 5 a./dm.

12. The process as claimed in claim 1 in which the current is passedthrough the electrolysis cell for anodic treatment of the object at acurrent density of 0.1 to 1 a./dm.

13. The process as claimed in claim 1 in which the current that ispassed through the electrolysis cell during anodic treatment has avoltage within the range of 10 to 80 volts.

14. The process as claimed in claim 1 in which the current that ispassed through the electrolysis cell during anodic treatment has avoltage within the range of to 30 volts.

'15. The process as claimed in claim 1 in which the electrolysis bathduring anodic treatment is maintained at a temperature within the rangeof 15 to 50 C.

16. The process as claimed in claim 1 in which the electrolysis bathduring anodic treatment is maintained at a temperature within the rangeof 18 to 25 C.

17. The process as claimed in claim 1 in which the anodic treatment ofthe object in the electrolysis cell is continued for a period within therange of /6 to 30 minutes.

18. The process as claimed in claim 1 in which the anodic treatment ofthe object in the electrolysis cell is continued for a period within therange of /2 to 15 minutes.

19. The process as claimed in claim 1 in which the metal salt which isdissolved in the electrolysis bath for the deposition of coloredparticles into the porous surface of alumina is a salt of a metalselected from the group consisting of copper, nickel and cobalt.

20. The process as claimed in claim 19 in which the metal salt isdissolved in the bath in an amount greater than 0.15 g./liter to anamount to form a saturated solution.

21. The process as claimed in claim 19 in which the alternating currenthas a density within the range of 0.5 to 5 a./dm.

22. The process as claimed in claim 1 which includes the step ofsubjecting the object to be colored to a surface treatment prior toelectrolysis treatment to modify the appearance of the colored coatingthat is formed.

23. The process as claimed in claim 1 in which the metal compounddeposited in the porous layer of alumina contains a compound of copper.

24. An object of aluminum or alloy of aluminum having a uniform coloredcoating produced by the method of claim 1.

References Cited UNITED STATES PATENTS 3,382,160 5/1968 Asada 204- NFREDERICK C. EDMUNDSON, Primary Examiner

